What follows is a discussion lab that I wrote for this past week’s WxChallenge forecast competition as part of Penn State University World Campus’ METEO 410 capstone class in weather forecasting. I’m sharing this to give folks a glimpse into the forecasting process we’ve been learning, and because this discussion lab garnered some plaudits from my instructor for providing a really good analysis with attention to detail.
Model Guidance
12Z NAM MOS and 18Z GFS MOS (model output statistics) today agreed on a high temperature of 46°F for Day 7 (06Z Thursday to 06Z Friday). 00Z Wednesday’s NBM run was forecasting 44°F. 18Z EKDMOS shows ~46°F in the 50th percentile, with 50°F in the 90th percentile, and 41°F in the 10th percentile.
Synoptic Set Up
By Thursday, the cold front of the occluded low that will bring precipitation Wednesday is forecast to have pushed through KCAR. During this frontal passage, winds will veer from the SSE towards the west. Winds are not forecast to be particularly strong, however, westerly winds would downslope a bit, enhancing wind speeds as well as warming temperatures a touch.
Typically, we’d expect temperatures to be cooler behind a cold front due to cold air advection (CAA). Checking dynamical model forecast 2-meter temperatures, there’s not really evidence of large temperature gradients around KCAR. Even though winds will be blowing from areas of cooler temperatures towards warmer temperatures, the lack of a large gradient and low wind speeds do not suggest strong CAA.
It’s worth noting that the cold front appears to have anafrontal characteristics (precipitation behind the front seen in the WPC surface forecast) – this has implications on cloud cover behind the front. Both sets of MOS guidance show overcast conditions throughout the day. Forecast soundings suggest the main effect of the cold front is a drying out of the layer between approximately 900 mb to 600 mb initially, though by 18Z Thursday this layer dry layer tops out 700 mb (NAM has a smaller dry layer, between 900 mb and 750 mb – not pictured). Outside of this dry layer, clouds appear likely both near the surface and also from the top of the dry layer to as high as 200 mb (300 mb in NAM). In fact, it appears that the column above the dry layer will be saturated, and precipitation will be falling at upper levels during periods of the day, which explains why the dry layer shrinks from the top down as moisture works its way down through the column.
Closing Thoughts
Because of the likelihood of persistent, seemingly thick overcast during peak heating, I’m hesitant to side with the MOS consensus of 46°F, which I think is too warm. Even if precipitation doesn’t reach the ground, evaporational cooling may still be a factor. I think NBM’s 44°F is reasonable given the current data. I wouldn’t go too low into the low-40s because of warming impacts of downsloping westerly winds and the lack of any strong CAA.
Results
Subsequent MOS runs actually trended up, as high as 49°F. However, because of the factors outlined above, I continued to hedge down from MOS guidance, and submitted a finalized forecast of 46°F on the day. The actual high ended up being 45°F. Because I hedged down, I was able to minimize my error points for the day and ended up climbing to the top of the class leaderboard.
